1. Field of the Invention
The present invention relates to an inkjet ink, an ink cartridge containing the inkjet ink, an inkjet recording apparatus which records an image by discharging the inkjet ink, an inkjet recording method for recording an image using the inkjet ink, and an image formed by the inkjet recording method.
2. Discussion of the Background
Inkjet recording methods have the following advantages over other recording methods:                (1) The recording process thereof is relatively simple compared to other recording methods and therefore full color images can be easily produced; and        (2) inkjet printers can produce high-resolution color images although they have a simple constitution.        
Dye-based inkjet inks in which a water-soluble dye is dissolved in water or a mixture solvent of water and an organic solvent have been typically used as inkjet inks. However, dye-based inkjet inks have a drawback in that the resultant images have poor light resistance, although the dye-based inkjet inks can produce color images having good clearness. Therefore, pigment-based inkjet inks in which one or more pigments such as carbon black and various organic pigments are dispersed have been investigated because the resultant images have a relatively good light resistance compared to those of the dye-based inkjet inks.
Such pigment-based inkjet inks are typically prepared by mechanically pulverizing and dispersing one or more pigments, such as organic pigments and carbon black, a dispersing medium and a dispersant. In this case, unless the pigments are finely and stably dispersed in the dispersion medium while having a small particle diameter, the resultant ink cannot form images having good transparency, high resolution and good color reproducibility. In addition, such an inkjet ink causes a clogging problem in that the nozzles from which the ink is to be discharged are clogged with the large pigment particles included in the resultant inkjet ink. However, in general there is a certain limit to the particle diameter of pigment particles in an inkjet ink which is prepared by a method utilizing a mechanical pulverization/dispersion operation. Even when an ink in which a pigment is finely dispersed can be prepared, the pigment ink tends to cause a problem such that the pigment agglomerates when the ink is preserved, i.e., the ink has poor preservability.
In attempting to solve the clogging problem and improve color reproducibility, various methods in which the pigment used for an ink is pulverized and dispersed so as to have a small particle diameter in the ink have been proposed. For example, published unexamined Japanese Patent Application No. (hereinafter referred to as JOP) 56-147863 discloses inks including a polymer dispersant such as styrene-acrylic copolymers or styrene-maleic acid copolymers. JOP 61-083267 discloses inks including a polymer dispersant such as condensates of sodium naphthalenesulfonate with formalin. JOPs 05-105837, 10-168367 and 10-088050 have disclosed inks including a surfactant such as polyethyleneglycol alkylphenyl ethers, sulfates of polyethyleneglycol alkylphenyl ethers, and phosphates of polyethyleneglycol alkylphenyl ethers. However, the pigments included in these inks have large average particle diameter, and thereby the inks have poor discharging stability and preservability. In addition, the resultant images have poor clearness.
Conventional inkjet printers produce full color images using (i.e., mixing) a yellow ink (Y), a magenta ink (M), a cyan ink (C), and a black ink (K). However, there is a limit to the color reproducibility of the resultant full color images when the images are prepared by mixing such yellow, magenta, cyan and black inks. In particular, secondary color images, i.e., red (R), green (G) and blue (B) images, have to be prepared by mixing two or more of the Y, M, C and K inks. Therefore, color images having good chromaticness cannot be produced.
In attempting to improve the color reproducibility of such secondary color images and prevent blurring of the secondary color images due to mixing of two or more inks, JOPs 08-244254 and 10-044473 have disclosed image forming methods using seven color inks, i.e., Y, M, C, K, R, G and B inks. However, when such seven inks are used, the number of the inkjet heads has to be increased, thereby causing a problem such that the manufacturing cost of the inkjet recording apparatus increases. In addition, it takes a long time and is troublesome to perform maintenance on such inkjet recording apparatus. Further, it takes a relatively long time to convert image signals to output signals, i.e., image processing time increases. Thereby, a problem in that the recording speed of the inkjet recording apparatus decreases occurs.
In general, images produced by pigment inks have relatively low brightness and chromaticness compared to images produced by dye inks, i.e., images produced by pigment inks have inferior to images produced by dye inks in color reproducibility. In particular, with respect to brightness and chromaticness of secondary color images, pigment inks are seriously inferior to dye inks. In attempting to solve this problem, WO99/05230 discloses a technique using Y, M, C, K, G and Or (orange) pigment inks. However, the color reproducibility of the inks are not satisfactory, and in particular the red color reproducibility is not satisfactory.
JOP 2001-192583 discloses a technique in that a combination of a specific dispersant and a specific additive is used to efficiently prepare an ink including a pigment having a small particle diameter in a short period of time. However, the average particle diameter of the pigment in the ink is on the order of 100 to 200 nm. In order to impart good discharging stability to an inkjet ink and to produce images having good chromaticness (i.e., clearness), the pigment in the ink has to have a particle diameter of from 10 to 50 nm. An ink in which such a fine pigment is dispersed cannot be produced by any conventional techniques.
As mentioned above, there is a limit to the particle diameter of pigment particles included in inks prepared by pulverization/dispersion methods. In addition, the particle diameter means an average particle diameter, and therefore large pigment particles are also included therein. When such large pigment particles are included at a high content, the ink tends to cause a clogging problem and therefore it is not preferable. In addition, even when pigment particles are finely pulverized, the pulverized pigment particles have new cleaved surfaces due to the pulverization operation, and thereby the surface area of the pigment particles is increased while the surface of the pigment particles is activated. Therefore, a problems such that the pigment agglomerates when the ink is preserved occurs, i.e., the ink has poor preservability.
Because of these reasons, a need exists for a pigment inkjet ink which has good discharging stability and preservability and which can produce good images having good chromaticness (good clearness).